Receivers used in various types of circuits are ideally able to handle large signals while maintaining good sensitivity for weak signals. For example, receivers configured to process return echo signals, such as ultrasound signals and radar signals, often encounter both large signals and small signals. An additional desirable feature for receivers is a quick recovery time from overload due to large signals exceeding the capabilities of the receiver. Another desirable feature for these systems is a low amount of power dissipation to reduce the heat produced by the system. Furthermore, there should be some protection against large signals that could possible damage the receiver.
With reference to FIG. 1, a circuit schematic of a receiving channel 100 of a prior art ultrasound system is presented. Receiving channel 100 generally includes a transducer section configured to transmit ultrasound excitation signals and receive a return echo signal. The return signal is propagated to a bridge section, which is biased to a certain level. The return signal is then propagated to an amplification section to bring the signal to a level where it can be processed by other devices. Typically, the signal is clamped to a predetermined level that does not overload the amplification section.
With further reference to FIG. 1, an exemplary embodiment of the present invention is illustrated. A transducer section typically comprises a transducer 102 configured to transmit ultrasound signals and receive a return echo. The signal used to excite transducer 102 and generate the high frequency sound pulses is applied through diodes 106 and 108. Diodes 106 and 108 are configured to steer the high excitation voltage into transducer 102, and block the relatively small return signals caused by the return echo received by transducer 102 from propagating to the rest of the circuit.
The bridge section receiving the return signal (i.e., the echoed signal) comprises a diode bridge comprising diodes 110, 112, 114, and 116 to help steer the current. The diode bridge is biased by a resistor 118 in series with an inductor 119, coupled to a positive power supply 111 and a resistor 120 in series with an inductor 121, coupled to a negative power supply 113. Diodes 122 and 124 limit the signal that propagates to the remainder of the circuit to +/− one diode voltage drop (e.g., 0.7 to 0.8 volts).
The signal from the diode bridge then propagates to low noise amplifier 126, voltage controlled amplifier 128, and post-amplifier 130. From this point, the signal can be processed by various systems to create a video image suitable for display on a video monitor.
With reference to FIG. 3A, an exemplary pulse transmitted to transducer 102 is shown. It can be seen that a short pulse with both positive and negative components is input to receiving channel 100, followed by a zero volt signal until the next pulse is due (in approximately 50 to 250 microseconds (μs)).
There are several potential problems with the above-described receiving channel 100. For example, there is relatively large power dissipation present. In order to have good sensitivity in the receiving channel, a high bias current may be used to lower the resistance of the diode bridge, in turn lowering the noise level. The voltage supplies 111 and 113 of the prior art system are typically configured to be +/−15 volts. Diodes 110, 112, 114, and 116 are typically biased with a relatively high current, e.g., 10 milliamps (“mA”). Thus, the power dissipation is approximately 300 milliwatts (“mW”). When multiple sensing channels are used, such a power dissipation is proportionately increased by the number of channels used. Such a power dissipation may result in various undesirable heat problems.
In addition, in the embodiment of FIG. 1, low noise amplifier 126 typically has a voltage gain of approximately 10. Since system 100, as a whole, is typically configured to output about 2 volts, it may be desirable to limit the input to low noise amplifier 126 to approximately 0.2 volts to prevent an internal overload. However, the input to low noise amplifier 126 is only limited by the turn-on voltage of diodes 122 and 124 (approximately 0.8 volts). Therefore, a signal greater than 0.2 volts but less than 0.8 volts will propagate to low noise amplifier 126, possibly resulting in an overload of amplifier 126.